A p-i-n diode, an avalanche photo diode and a photo transistor, such as a bipolar transistor, which are employed as photo detectors in conventional photo couplers posses many defects.
The p-i-n diode and the avalanche photo diode are two-terminal elements, and hence have the drawback that they have no function of isolation from the next-stage element. The p-i-n diode and the avalanche photo diode are supplied with a relatively high voltage for operation by irradiating their depletion layers with light. The avalanche photo diode has the serious shortcoming of producing much noise which is due to light that results in an avalanche multiplication.
The photo transistor using a bipolar transistor has a gain as small as 100 or so and has a large base resistance, and hence it possessses the defect of very low-speed operation.
FIGS. 1A and 1B show examples of the arrangements of conventional photo couplers.
FIG. 1A illustrates a photo coupler which employs a GaAs light emitting diode 1 as a light emitting element and a p-i-n diode 2 as a photo detector.
Reference numeral 3 indicates a packaging case in which the light emitting diode 1 and the p-i-n photo diode are housed. There are times when the photo detector 2 is an avalanche photo diode. In operation, an input signal I.sub.1 flows in the light emitting diode 1 to generate light h.nu. and a current I.sub.2 flows in the p-i-n photo diode 2, developing an output signal across a load resistor R.sub.L. This arrangement is defective in that a relatively large bias source is needed and in that a special consideration must be paid to its coupling with a circuit of the next stage.
FIG. 1B shows a conventional photo coupler using a bipolar transistor 4 as the photo detector. The light h.nu. from the light emitting diode 1 is amplified by the bipolar photo transistor with its base floating. The bipolar transistor has a large base resistance and has the defect that the operating speed is low.
As described above, the conventional photo couplers posess serious drawbacks in practical use.